Acoustic device



De.17,194o.y R, A HACKLEY 2,224,909

ACOUSTIC DEVICE Filed July 27, 1939 3 Sheets-Sheet l /faH/zoA/vwz Y srt/EEP a,.o.

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EMPL/WER a v y inventor Dec. 17, 1940. R, A; HACKLEY 2,224,909

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l l I .57 69 caw/107777017 BHI/SH Dec. 17, 1940. R. A. HACKLEY ACOUSTIC DEv'IcE Filed July 27, 1939 5 Sheets-Sheet 3 Fira.

'ro om DEFL'V.' neergang ww/@DE )my TUBE 110V a c Isnven oi' Begin d H Hacf Z et Patented Dec. 17, 1949 UNITED STATES ACOUSTIC DEVICE Reginald A. Hackley, Bala-Cynwyd, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application July Z7, 1939, Serial No. 286,752

8 Claims.

T' is invention relates to acoustic devices and more particularly to a cathode ray high speed recorder for making acoustic measurements.

In acoustic work, it is highly desirable to study response as a function of frequency to determine the performance of an acoustical device. vNumerous recording systems have been developed. While it has been proposed to use a cathode ray oscillograph, it was found that the trace had lo to be repeated sufficiently rapid to give an' appearance of a stationary image. This rate of repetition was determined by the retentivity of the cathode ray tube screen. In view of the fact that the retentivity of a conventional screen was IIS only a fraction of a second, it was necessary to have an extremely high scanning rate. This, in turn, brought about the difliculty that as the low frequency portion of the response curve, the

` trace followed the form of the individual partial cycles of the driving voltage rather than being made up of a plurality of points which were the resultant of several complete cycles.

One of the objects of this invention is to provide means for overcoming the aforementioned difficulty. Another object is to provide an acoustic recording device in which a cathode raytube with a persistent screen is used whereby a single trace can be observed for several minutes. A

further object is to provide an acoustical recording device in which the scanning rate is low enough so that each point on the trace repre-- sents the resultant of several complete sound cycles. `A further object of this invention is to provide means in an acoustical recording device whereby any desired portion of a frequency response curve may be traced on a persistent screen of a cathode ray tube.

The invention will be described by referring to the accompanyin'gdrawings in which Figure l is a schematic representation of one embodiment of the invention; Figure '2 is a detailed plan `view of one' of th'eelementsfof the-invention;l and Figure 3 is a'. schematiccircuit diagram of the motor reversing device and switching circuit employed in the invention. Similar reference numerals will be used to indicate similar elements in the several drawings.

.Referring to Fig. 1, a beat frequency oscillator I, the frequency control of which is rotated by a motor v3, is connected to the loudspeaker 5 under test. A microphone or other suitable pickup device l is suitably placed to receive the sound waves emanating from the loudspeaker. 'I'he microphone output is connected to an amplifier 55 9. The amplifier output is applied to a logarithmic detector II. The -output from the detector II is impressed uponaD. C. amplifier I3. The output of the D. C. amplifier is connected through a low pass filter I 6 to a D. C. amplifier I 1. The output of the D. C. amplifier is connected to the vertical deflecting electrodes of a cathode ray tube I9, which includes a screen having a retentivity of several minutes. The horizontal deflecting electrodes of the cathode ray` tube are connected to a horizontal voltage sweep source 2| which is driven by the motor 3. The motor also operates a reversing control means 23. y y

In the operation of the foregoing device, beat frequency currents,.beginning, for example, at the lowest audio frequency and gradually increasing to the highest audio frequency, are applied to the loudspeaker 5. Thus, the speaker is energized throughout the desired frequency range.

At the same time that the frequency is increased, voltages from the horizontal sweep source are applied tothe cathode ray tube to deflect the ray as a function of the frequency applied to the speaker. The speaker radiates sound waves which actuate the microphone to establish thereby voltages corresponding to the sound waves.

'I'hese voltagesA are amplified and detected. The detector is preferably arranged so that the output voltage varies logarithmieally with the input voltage. This is desirable because the input amplitude variations will then be indicated in decibels. The detector output-may be amplified to any desired degree and since this output lwill include, in addition to the direct current component, a. double frequency of the alternate ing current component, it is desirable to remove the alternating current component which is done by means of the low pass filter. The time constant of the filter is made low enough to pass the amplitude variations in the direct current component result from the variations in the acous- 'tical output of the-loudspeaker.

Thenal direct current component, after further ampliflcation, is applied to deflect vertically the cathode ray. The resulting movements of foregoing device are known tot-hose skilled in,

, drive plate 4 I.

'commutator segment.

the art, a more detailed' description of the frequency sweep and motor reversing circuits will be helpful. Referring to Fig. 2, the motor 3 is connected through a suitable flexible coupling 25 to a reduction gear drive 21. The reduction gear drive operates a pulley 29 and a bevel gear 'I'he pulley is connected by means of a belt 33 to a second pulley 35 which is mounted on the control shaftof the beat frequency oscillator The bevel gear 3| engages a second bevel gear 31rwhich is mounted on one end of a shaft 39; the other end of the shaft is connected to a friction drive plate 4|. A suitable pinion wheel 43, preferably including a rubber tire 45. engages the The pinion wheel` is mounted ony a shaft 41 which is coupled to the potentiometer or, horizontal sweep device 2|. 'I'he coupling includes a slidable member 49 which carries a rack 5|. The rack is moved back and forth by means of a gear and worm 53. The position of the pinion` wheel 43 on the driving plate 4| determines the ratio of movement of the beat frequency oscillator control shaft with respect to the potentiometer shaft. A

Intermediatel the second bevel gear and the friction drive plate is disposed a reversing control 23. This control includes a pair of movable These brush holders are adjusted respectively by means of worm and gear devices 59-B|. The brushes engage a commutator which is composed of two segments, one of the segments 63 covers approximately 40 degrees; the other segment B5 covers approximately 320 degrees. The two aforementioned commutator segments are connected together by means of a third segment 61 which covers 360 degrees. A fixed brush 69 engages the 360 degree As will hereinafter appear, the movable brush holders 55-51 may be adjusted with respect to each other so that any desired range of beat frequencies may be obtained from the oscillator l.

. The details of the motor reversing control and the horizontal deection potentiometer are shown in Fig. 3. 'Ihe motor 3 is preferably a series wound motor which may be readily reversed by reversing its field coil 1| with respect to its armature 13. The armature iscoupled, as already described, to the potentiometer 2| and to the shaft 39 wichh operates the commutator 63- 65-51. The fixed brush 89 is connected to one terminal of the `110 volt A.C. line. The other terminal of the line is connected through the switch15 to one of the field windings which is terminated at a fixed contact of a relay 8| operated double pole-,double throw switch 11. The

other xed contact of the D. P. D. T. switch is connected to the other field winding. 'I'he upperv two contacts of the switch 11 are connected to the armature'13'through a rheostat 19. The lower fixed contacts of the switch 11 are also connected to the armature 13` but in a reverse sense by a also connected through an auxiliary switch contact 85 to the other movable brush 51.

To prevent the cathode ray from retracing the L resultant curve. a beam return relay 81 is employed. relay is connected in parallel with the motor reversing relay 8|. The beam return relay includes a movable contact arm 89 and a pair of fixed contacts 9|93. The operation of the relay 81 applies a potential which immediately restores the cathode ray beam to its original position. The original position fincidentally is adjusted by means of a pair of rheostats 95-91 which are connected in series with4 either fixed `terminal of the potentiometer 2|. This adjustof the commutatoi'. At this point, the motor reversing relay willbe deenergized and will open the auxiliary switch 85 as well as reverse the D. P. D. T. switch 11. At the same time the motor will reverse its direction of movement; and will continue to rotate until the motor reversing relay `is again energized by the connection of the brush 55, thus completing the cycle. When the motor reversing relay 8| is energized, the beam return relay is also energized, therebyapplying the horizontal deecting voltage to the horizontal deflecting electrodes of the cathode ray tube. When the motor reversing relay is de-energized, the movable contact 89 of the beam return relay 81 engages the fixed contact 93 and thereby returns the beam to its original position. As thus arranged, the horizontal" deflections of the'cathode ray are always traced from the lowest initial voltage to the highest final voltage corresponding to the lowest frequency of the oscillator to the highest frequency. This type of movement overcomes all the difiiculties of back-lash and permits substantially exact retracing of any graph. Furthermore, the adjustment of the relative positions of the movable brushes 55-51 provides a means for determining the range of beat frequencies obtained. As previously explained, the rack adjustment of the movable pinion wheel determines the sweep range of the cathode ray. Thus arranged, the device may be used to explore either the entire range of audio frequencies or any portion thereof, or a desired portion may be explored over the entire sweep circuit. Synchronization of the beat. frequency oscillator control and the potentiometer, or sweep control is accomplished by employing stops 99.

I claim as my invention:

1. In an acoustic device, a cathode ray tube including a persistent screen and beam deecting means. a source of current of variable frequency,L means'for defiecting the beam of said cathode ray tube alongl one coordinate in synchronism with said variation in one direction and for returning said beam rapidly in the other direction, means for applying said variable currents to an object to be tested to thereby establish sound waves. means responsive to said' sound waves for converting said waves into electrical energy, and means for applying said energy to deflect .said beam along another coordinate.

2. In'an acoustic device, a cathode ray tube including a persistent screen and beam deecting means, a source of current of variable frequency, means for deilecting the beam of said Vcathode ray tube along one coordinate in synthe sound, and means chronism with said variation in one direction and for returning said beam rection, means for applying said variable currents to an. object to be tested to thereby establish sound waves, means responsive Vto said sound waves for converting said waves into electrical ,energy bearing a vlogarithmic relation to alongpanother coordinate.

3. In a device of the character ofv claim 1, a tllter for removing undesired components established by the` c`onversion oi' said sourd energy into said applied electrical energy, said filter to deect said beam having a time constant which does notsubstany desired components.

4. In a device of the character of .claim 1, means for-selecting any desired band of variable frequency currents, and means for adtially modify the vjusting and synchronizing said beam deecting means`-so thatv said deflections along said one direction may be made to correspond to the selected band. y

5. In an acoustic device, a source of current of variable frequency, a motor, means coupling said motor to said sourceso that said frequency may be varied thereby, a reversing switch connected to said motor, means for operating said switch, a commutator driven by said motor, a

pair of brushes relatively adjustably engagingsaid commutator and connected to said switch operating means, and means for limiting the' operation of said switch operating means to a function of said relative brush adjustment.

6. In an acoustic device, a cathode ray tube including beam denectins means, a source of current of variable frequency, a motor, means rapidly in the other dil 4 erating said switch,

for applying said energy.

coupling said motor to said source so'that said frequency may be-varied thereby, a reversing switch connected to said motor, means for op-v acommutator driven by said motor, a pair of brushes relatively ad-` justably engaging said commutator and con-l nected to said switch operating means, means for limiting the operation of said switch oper-` ating means to correspond to the relative adjustment of said brushes, anda sweep voltage circuit connected to said beam deflecting means and operated by said motor. Y

7. In an acoustic device, a cathode ray tube including beam defiecting means, a source of current of variable frequency, a motor, means coupling said motorvto-said source so that said frequency may be varied thereby, a reversing switch connected to said motor, means for operating said switch, a commutator driven by said motor, a pair of brushes relatively adjustably engaging said commutator and connected to said switch operating means, means for limiting the operation rof said switch operating means to correspond to the relative adjustment of said brushes, a sweep voltage circuit connected to said beam deflecting means and operated by said motor, -and means for adjusting said sweep independently of said frequency variation so that any portion of the sweep voltage may be apy plied to the beam deecting means.

to the respective terminals ,of said sweep voltage circuit for centering REGnrALD A. HACK-LEY.

the beam of said tube. 

